(1) Field of the Invention
The present invention relates to a method for repairing turbine engine components made from a nickel-based superalloy.
(2) Prior Art
The welding of highly alloyed nickel-based materials, also known as superalloys, is difficult as judged by non-destructive inspection rejections which require that defective welds be recycled, i.e. re-welded, re-heat treated, re-inspected, etc. This situation especially applies to cast superalloys having high titanium and aluminum concentrations in addition to other elements, such as carbon and boron, which can reduce weldability.
It is known in the prior art to repair cast nickel-based superalloy gas turbine engine cases using a replacement flange and electron beam (EB) welding. The replacement flange is typically formed from a wrought nickel-based superalloy. Some electron beam welds used to join these two components together experience a controlled amount of microcracking which typically occurs in heat affected zones (HAZ) and sometimes in the weld area itself. These subsurface defects can propagate to the surface as a result of solidification shrinkage strains and/or post-weld heat treatments intended to lower or eliminate weld residual stresses. Such surface cracks are detectable by fluorescent penetrant inspection (FPI) techniques only if they are large enough. The same strains can also propagate welding-related cracks and make them detectable to radiographic inspection as either sub-surface or surface-connected cracks.
In the case of electron beam welding replacement flanges onto engine-run hardware, the post-weld heat treatment often requires a full heat treatment (solution, stabilization, and/or aging) to restore the component's mechanical properties. Cast cases can also have localized cracks weld repaired during original manufacturing or after engine operation. In this situation, a gas tungsten arc welding (GTAW) process may be selected rather than electron beam welding to reduce manufacturing expenses and repair turn time. GTAW processes, by their nature, introduce significantly more heat into the cast material in comparison to electron beam welding. This situation produces both wider welds and heat affected zones. The additional energy increases cracking frequency and size. The need for repairing turbine engine components, such as original equipment cases, prior to engine operation, results from the presence of various casting defects such as cracks, porosity, and casting dross. Engine-operated cast case repairs are usually driven by cracks which exceed operational acceptance limits. Such cracks can result from operating stresses/temperatures or may be casting defects which propagate to detectable size during engine operation and/or repair thermal cycles.
Both EB flange replacement and GTAW crack weld repairs require that the welding source demonstrate its ability to produce acceptable welds. Defects visible by fluorescent penetrant inspection (FPI) are not allowed. Defects detected by radiography are allowed according to size and frequency limits. However, destructive evaluation limits are also present. They pertain to defects which can only be detectable through metallographic evaluation. Metallographic limits allow the presence of microcracks but again limit their size and frequency.
There is a need for a method which allows repairs to be performed with less need for re-welding using electron beam welding or other welding methods (e.g. GTAW, plasma arc).